1. Field of the Invention
The present invention relates to a die for forming a resin layer as an intermediate layer of an optical disk or a coat layer of a front surface or a rear surface of the optical disk, a method of manufacturing an optical disk, and an optical disk.
2. Related Art of the Invention
A general structure of a conventional optical disk will be explained. In recent years, there have been remarkable increases in a recording density and a capacity of optical disks used for various applications for information recording such as audios, images, and computers. Products such as CDs (compact disks) and DVDs (digital versatile disks) have been produced. In general, such optical disks are constituted by stacking a recording layer, a reflection layer, and the like on a substrate and further providing a protective layer. In general, a PC (polycarbonate) substrate is used for the substrate.
In particular, in recent years, an optical disk having a large capacity is demanded. The development of a one-side multilayer disk provided with two or more recording layers on one side of a disk has been advanced. A next-generation DVD having a diameter of 120 mm and a recording capacity of several gigabytes or more has been realized. In such a next-generation DVD, a recording density is increased by a high NA (Numerical Aperture: the number of openings of a lens). A focal length of the lens is reduced as the NA is increased. Thus, it is necessary to bring the recording layers closer to the lens and it is demanded to reduce thickness of a cover layer for protecting the recording layers. In a lens having a higher NA, blur and aberration tend to occur. Thus, high uniformity is required for qualities and thicknesses of layers (hereinafter referred to as film qualities and film thicknesses) of not only the cover layer but also in planes of respective layers forming the optical disk such as an intermediate layer.
A structure of a next-generation one-side two-layer disk is shown in FIG. 19. In the next-generation one-side two-layer disk, a first recording layer 102 having a recording track 102a is formed on a disk substrate 101, a second recording layer 104 having a recording track 104a is formed on the first recording layer 102 via an intermediate layer 103, and a cover layer 105 is formed to cover the second recording layer 104. A rear-side coat layer 106 is formed on a rear surface of the disk substrate 101.
A light beam is irradiated on the next-generation one-side two-layer disk having such a structure from the side of the cover layer 105 and recording and reproduction or only reproduction of information in the first and the second recording layers 102 and 104 is performed.
A method of manufacturing the disk will be briefly explained.
As shown in FIG. 20A, the first recording layer 102 is formed on the disk substrate 101 on which a pattern of a recording track is formed. An ultraviolet-curing adhesive 107 (hereinafter referred to as adhesive 107) is applied over this first recording layer 102 as shown in FIG. 20B. In parallel with the application of the adhesive 107, as shown in FIG. 20D, ultraviolet-curing resin 109 is applied over a stamper 108 on which a pattern of a recording track is formed as shown in FIG. 20C.
Subsequently, as shown in FIG. 20E, the disk substrate 101 and the stamper 108 are stuck together and pressed with coated surfaces thereof opposed to each other under vacuum. An ultraviolet ray is irradiated on the disk in that state to harden the adhesive 107 and the ultraviolet-curing resin 109, whereby the intermediate layer 103 is formed. Thereafter, as shown in FIG. 20F, the stamper 108 is peeled off. As shown in FIG. 20G, the second recording layer 104 is formed on the intermediate layer 103, onto which the pattern of the stamper 108 is transferred, and the cover layer 105 and the rear-side coat layer 106 are formed. In this case, a method of application by nozzles, a spin coat method, a sheet sticking method, or the like is used for the application and formation of the adhesive 107, the ultraviolet-curing resin 109, the cover layer 105, and the rear-side coat layer 106.
As a representative method of the method of application by nozzles, which is one of the conventional methods, there is a die coat method (see, for example, Japanese Patent Publication No. 5-5548 (pages 3 to 11). Schematic diagrams of a coating device 160 used for the die coat method are shown in FIGS. 21 and 22.
FIG. 21 is a top view of the coating device viewed from a surface on a side from which a resin layer (equivalent to the ultraviolet-curing adhesive 107 in FIG. 19) is applied when the resin layer is applied over the disk substrate 101 having the first recording layer 102 formed thereon (hereinafter referred to as disk substrate with recording layer 121), which is mounted on a rotation table 159. FIG. 22 is a side sectional view of the coating device 160 viewed from an arrow A side in FIG. 21. As shown in FIGS. 21 and 22, in the die coat method, while a die 127 which discharges paint is rotated in an arrow direction relatively to the disk substrate with recording layer 121, which is a coating object, around a central axis 121a of the disk substrate with recording layer 121, resin is discharged from the die 127. This makes it possible to form a resin layer on the disk substrate with recording layer 121.
Diagrams of a conventional die used when a resin layer is coated by such a die coat method are shown in FIG. 23 to FIGS. 25A and 25B.
FIG. 23 is a perspective view of the conventional die 127. The die 127 is divided into an upper block 111 on an upper side and a lower block 112 on a lower side of the die 127 on the basis of an alternate long and short dash line BB′ in FIG. 23. FIG. 24A is a perspective view of only the lower block of the conventional die. FIG. 24B is a plan view of the lower block viewed from a D side in FIG. 24A. FIG. 25A is a perspective view of a state in which the upper block and the lower block of the conventional die is intentionally separated vertically. FIG. 25B is a plan view of a discharge port viewed from an E side in FIG. 25A.
As shown in FIG. 23 to FIGS. 25A and 25B, the conventional die 127 includes the upper block 111 and the lower block 112. A shape of this lower block 112 is, as shown in FIG. 24A, substantially a trapezoidal prism. The lower block 112 is mounted such that trapezoidal surfaces 112a and 112b having an upper side longer than a lower side are arranged on sides. A manifold 113 is formed from one trapezoidal surface to the other trapezoidal surface in an upper surface 112c of the lower block 112. In addition, a paint supply path 110 is formed from a side 112d other than the trapezoidal surfaces 112a and 112b to the manifold 113 in the lower block 112. A surface opposed to this side 112d is assumed to be a side 112e. A rear lip 120 projecting from the side 112e is formed at an end on the side 112e side of the upper surface 112c. The shape of the lower block 112 being substantially a trapezoidal prism means that the shape of the lower block 112 is in a range recognized as a trapezoidal prism under the commonly accepted idea.
As shown in FIGS. 23 and 25A, a shape of the upper block 111 is also a trapezoidal prism substantially the same as the shape of the lower block 112. However, the manifold and the paint supply path 110 are not formed in the upper block 111. This upper block 111 is mounted on the lower block 112 such that an upper surface and a lower surface thereof are opposite compared with those of the lower block 112. A front lip 119 is formed in a portion of the upper block 111 opposed to the rear lip 120 of the lower block 112.
Between the lower surface of the upper block 111 and the upper surface of the lower block 112, a slit 114 is formed from the position of the manifold 113 to the position of the rear lip 120. This slit 114 is formed from the trapezoidal surface 112a side to the trapezoidal surface 112b side (see FIG. 25A). A portion between the front lip 119 and the rear lip 120, which is a part of this slit 114, forms a discharge port 118. Width 117 of this discharge port 118 from the trapezoidal surface 112a to the trapezoidal surface 112b is coating width in which paint is applied. Although not shown in the figure, sidewalls are formed on the trapezoidal surfaces (including the surfaces 112a and 112b) shown in FIG. 23 such that the paint does not leak from ends of the manifold 113 and the slit 114.
In the conventional die having the structure shown in FIG. 23, a resin liquid supplied from the paint supply path 110 is spread and distributed in the width 117 direction by the manifold 113, passes through the slit 114 formed between the upper block 111 and the lower block 112, and is discharged from the discharge port 118.
In the coating device 160 shown in FIGS. 21 and 22, the die 127 is arranged such that the discharge port 118 is opposed to the plane of the disk substrate with recording layer 121. An area on which the resin is applied is denoted by reference numeral 150, an outer peripheral side end of the area 150 is denoted by reference sign 150a, an inner peripheral side end thereof is denoted by reference sign 150b, and width thereof is denoted by reference sign 150h. A position of the discharge port 118 of the die 127 corresponding to the outer peripheral side end 150a is assumed to be an outer peripheral side end 118a and a position thereof corresponding to an end on the inner peripheral side of the area 150 is assumed to be an inner peripheral side end 118b. Length of the width 117 is equivalent to the width 150h of the area 150 on which the resin is applied (see FIG. 23 to FIGS. 25A and 25B).
In order to apply the resin over a disk-like coating object at uniform thickness with the method shown in FIGS. 21 and 22, since peripheral speed at the time when the coating object rotates is higher on an outer peripheral side than on an inner peripheral side of the coating object, it is necessary to discharge a larger quantity of resin at the outer peripheral side end 118a compared with the inner peripheral side end 118b of the coating object in a coating width direction according to the peripheral speed. When a resin layer having a coating width of 40 mm is formed on a disk having a diameter 120 mm, which is the present standard of optical disks, a discharge quantity three times as large as that at the inner peripheral side end 118b is required at the outer peripheral side end 118a. 
Moreover, the discharge quantity is affected by a pressure loss in the slit portion and a pressure loss ΔPs is in a proportional relation with a slit length 116 (Ls) (ΔPs∝Ls). Thus, in order to increase the discharge quantity, the pressure loss ΔPs only has to be reduced, that is, the slit length 116 (Ls) only has to be reduced.
Thus, when a difference is caused in the discharge quantity in the coating width direction, a slit length in a portion where it is desired to set the discharge quantity smallest only has to be set largest and a slit length in a portion where it is desired to set the discharge quantity largest only has to be set smallest.
Therefore, in the structure of the conventional die, as shown in FIG. 21, the slit length 116 (Lsb) at the inner peripheral side end 118b is larger than the slit length 116 (Lsa) at the outer peripheral side end 118a of the coating object. The slit length 116 (Ls) increases from the outer peripheral side end toward the inner peripheral side end. A difference is caused in resistance against flow of the resin in the width 117 direction by this structure to discharge a larger quantity of the resin at the outer peripheral side end compared with the inner peripheral side end of the coating object.
A slit gap 115 (hs) of the conventional die is set to be identical in any position in the width 117 direction in the area of the width 117 as shown in FIGS. 25A and 25B.
The method of forming a resin layer by the die coat method has been described. However, when the spin coat method of dropping resin on a disk substrate and, then, rotating a disk to spread the resin over the surface of the disk and obtain a film is used, it is also possible to form a resin layer. When the sheet sticking method is used, it is possible to form a resin layer by transferring a resin film formed as a sheet instead of applying resin.